TECHNICAL FIELD
[0001] The present invention relates to a nanophosphor used for a display and a nanophosphor
composite used for vital staining observation, antigen-antibody reaction site observation,
and protein observation.
BACKGROUND
[0002] Over recent years, in II - VI group semiconductors such as ultrafine particles, represented
by Si and Ge, or porous silicon, much attention has been paid to the fact that the
nanostructured crystal thereof exhibits specific optical characteristics. Herein,
the nanostructured crystal refers to a crystal particle of a particle diameter of
several nm, and is generally referred to as a nanocrystal.
[0003] When the case in which a II - VI group semiconductor features a nanostructured crystal,
as described above, is compared to the case in which the same features a bulk crystal,
excellent light absorption characteristics and light emitting characteristics are
expressed in the case of featuring the nanostructured crystal. It is conceivable that
a II - VI group semiconductor featuring a nanostructured crystal has a large band
gap due to its exhibition of a quantum size effect, as compared to the case of featuring
a bulk crystal structure. Namely, it has been thought that the band gap is widened
by the quantum size effect in the II - VI group semiconductor featuring a nanostructured
crystal.
[0004] Incidentally, various phosphors have been used for displays for TV sets.
[0005] The particle diameters of phosphors currently used for displays for TV sets are several
microns (3 - 10 µm). Over recent years, a variety of displays have been developed,
and much attention has been paid to plasma displays (PDPs), field emission displays
(FEDs), electroluminescence displays (ELDs), and SEDs (surface-conduction electron-emitter
displays), especially from the viewpoint of realizing thinner-type displays.
[0006] Of these, in the FEDs, it is necessary to reduce the electron beam voltage when employing
the thin type display. Since it is necessary to apply a high voltage to the microchip
and the diamond thin film used for an electron-emitting source to carry out electron
emission, an advanced vacuum technology is required to prevent breakage and also a
problem of excessive power consumption has been noted. Accordingly, designing for
lower voltage requirement has been urgently sought, and thus investigations toward
realizing an electron-emitting source employing a material such as a carbon nanotube
have been conducted.
[0007] However, when a phosphor of a particle diameter of several µm, as described above,
is used for a thin-type display, light emission is not adequately realized due to
the low electron beam voltage.
[0008] Namely, in such a thin-type display, it has so far been impossible to sufficiently
excite a conventional phosphor. The reason is that since the crystal of a conventional
phosphor is relatively large, no irradiated electron beams can reach the emitting
portion of the light emitter. Namely, when a conventional phosphor of a particle diameter
of several µm is used for a thin-type display, light emission has not been adequately
realized. Therefore, it can be said that a phosphor excitable at a low voltage is
suitable for a thin-type display, especially an FED. As a phosphor satisfying such
conditions, II - VI group semiconductors featuring a nanostructured crystal, as described
above, can be exemplified.
[0009] However, in the nanostructured crystals studied so far, there have been noted problems
of insufficient luminance and also occurrence of nonuniform luminance due to poor
size distribution resulting from aggregation, as well as due to emission killers caused
by excessive crystal surface defects (please refer to Patent Documents 1 - 4).
[0010] Further, in the biotechnology field, such a method has been conventionally employed
that in investigations of virus or enzyme reaction or in clinical assays, a fluorescent
substance composed of organic molecules is used as a marker, and fluorescence emitted
via UV irradiation is measured using an optical microscope or optical detector. As
such a method, for example, an antigen-antibody fluorescence method is widely known.
[0011] In this method, an antibody, to which a fluorescence-emitting organic phosphor bonds
(this is called a specific bonding substance), is used. Since antigen-antibody reaction
is extremely selective, the location of an antigen can be determined based on fluorescence
intensity distribution.
[0012] Incidentally, in this field, over recent years, there has been an increasing demand
to investigate more precise antibody distribution via observation of a substance less
than about 1 µm. To realize this, there has been no alternative but to depend on optical
microscopes.
[0013] In electron microscope observation, an image is observed via the difference in electron
beam reflectance or transmittance between specimens. Therefore, when an antibody is
observed with an electron microscope, molecules containing iron or osmium having a
relatively large atomic weight, or gold colloids of a size of about 1 - about 100
nm have been currently used as markers for the antibody. For example, when a gold
colloid is used as a marker, a composite of protein A and the gold colloid is allowed
to bond to an antibody. Since this antibody bonds to a corresponding antigen via an
antigen-antibody reaction, the localized site of the antigen can clearly be determined
via observation of the location of the gold colloid on the specimen. Further, when
at least 2 types of gold colloids of different sizes are allowed to bond to plural
types of antibodies, it is also possible to simultaneously observe plural antigens.
However, this method is disadvantageous in that colloids may overlap each other during
observation, and also it is difficult to conduct quantitative determination only via
measurement of the number of colloids.
[0014] Further, it is also difficult to observe a cathode luminescence image using the above
organic phosphor as a marker. Namely, organic phosphors naturally exhibit relatively
low emission efficiency and in addition, emission performance is decreased due to
the tendency of breakage of molecular bonds of dyes via electron beam irradiation,
whereby light emission tends to be markedly weakened after a single scan, resulting
in commercial non-viability.
[0015] Still further, these organic phosphors exhibit poor stability during storage, leading
to excessive degradation. As phosphors composed of organic molecules, polystyrene
spheres of a particle diameter of several tens of nm emitting light of red, green,
or blue are known, in addition to organic fluorescent dyes, but the exactly same problems
as described above exist.
[0016] In contrast, inorganic phosphors are stable to UV irradiation and electron beam irradiation,
resulting in less degradation. However, phosphors industrialized for TV sets or lamps
commonly feature a size of at least 1 µm, being unable to be employed, as such, as
phosphors for antigen-antibody reaction. Therefore, to decrease the particle diameter,
pulverizing phosphors or etching with an acid has been explored and carried out, but
via these methods, since the ratio, occupied by a non-emitting layer coating an individual
particle surface, increases, emission efficiency tends to markedly decrease.
Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter
referred to as JP-A) 2002-322468
Patent Document 2: JP-A 2005-239775
Patent Document 3: JP-A 10-310770
Patent Document 4: JP-A 2000-104058
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0017] In view of the above conditions, the present invention was achieved. An object of
the present invention is to provide a nanostructured phosphor and a nanophosphor composite
exhibiting excellent emission luminance.
MEANS TO SOLVE THE PROBLEMS
[0018] The above object of the present invention can be achieved via the following constitutions:
Item 1. A nanophosphor comprising a nanostructured inorganic crystal having a particle
diameter of 2 nm - 10 nm,
wherein the nanostructured inorganic crystal has a dispersion ratio of luminance intensity
distribution of 5% -
20%.
Item 2. The nanophosphor as described in item 1 above, having a half-value width in
an emission spectrum of 2 nm - 20 nm.
Item 3. A nanophosphor composite comprising the nanophosphor as described in item
1 or 2 above and an organic coating material covering the nanophosphor,
wherein a particle diameter of the nanophosphor composite is 5 nm - 15 nm.
[0019] Namely, the inventor of the present invention has conducted diligent investigations
to solve the above problems of a phosphor featuring a nanostructured crystal, and
has found that nonconventional, enhanced luminance characteristics were realized in
such a manner that the luminance per particle of a nanostructured crystal was determined,
and the dispersion ratio, on evaluation of the luminance intensity distribution, was
controlled within the range of a prescribed value of the present invention. It is
presumed that since the luminance distribution is in a monodispersion direction, the
compositions, surface levels (attributed to lattice defects), and crystalline properties
among particles are extremely close to one another, and then the quantum confinement
effect is greatly exerted in the entire phosphor. Especially in a dope-type nanostructured
crystal incorporating an activator serving as an emitting center, it is presumed that
the activator distribution per particle is similar among the particles, and also an
activator in one particle exhibits uniformly dispersed distribution, resulting in
enhanced quantum efficiency.
EFFECTS OF THE INVENTION
[0020] The nanophosphor and the nanophosphor composite according to the present invention
produce excellent effects in emission luminance.
BEST MODE TO CARRY OUT THE INVENTION
[0021] The present invention will now be detailed.
[0022] The dispersion ratio of emission luminance is expressed by a numerical value (%)
obtained by multiplying a value, obtained by dividing the standard deviation by the
average luminance, by 100, being an index for the monodispersion degree of the luminance
intensity of phosphor particles to represent the degree of variation from the average
luminance. The dispersion ratio of luminance intensity of the present invention is
5% - 20%, and preferably 5% - 15%.
[0023] To obtain luminance distribution, the luminance per particle needs to be determined.
A microscopic region is excited using a near-field microscope, and light emission
is detected by positioning an optical fiber on the tip of a cantilever to obtain the
luminance per particle. In the present invention, the dispersion ratio of luminance
intensity is determined by measuring 100 particles.
[0024] A half-value width corresponds to a spectrum width at half the maximum emission intensity.
The half-value width is 2 nm - 20 nm, and preferably 5 nm - 15 nm. The emission spectrum
of the present invention represents the average value when the emission spectra of
100 particles are determined, similarly to the case of the determination of the luminance
distribution as described above.
[0025] The nanophosphor composite of the present invention refers to a complex material
(a composite) prepared by coating an organic substance exhibiting bioaffinity or an
organic substance having a bio-adsorptive functional group on a probe phosphor to
impart bioaffinity (or so as not to induce living body-excreting reaction) when a
phosphor probe is introduced into a cell for vital staining observation, antigen-antibody
reaction site observation, or protein observation using an electron microscope or
a fluorescence microscope. For application to various types of cells, the particle
diameter of the probe is preferably as small as possible, and also a composite featuring
at most 15 nm has been greatly demanded. To realize this, it has been found that a
nanostructured crystal of a size of 2 - 10 nm is suitable as described in the present
invention, and a nanostructured crystal, featuring uniform luminance distribution,
exhibits high adsorptive properties to a coating material, resulting in a small coating
thickness, whereby the composite size can be controlled to be of a small particle
diameter.
[0026] As the coating material, there can be exemplified ethylene glycols, water-soluble
polymers incorporating a functional group exhibiting affinity to body tissues such
as a carboxyl group or an amino group, and active agents (TOPO: trioctylphosphine
oxide and TOP: trioctylphosphine). These may be used in combination or individually.
EXAMPLES
[0027] The present invention will now specifically be described with reference to examples
that by no means limit the scope of the present invention.
Examples 1 - 12
<<Preparation of Phosphors>>
[0028] In the examples, ZnS and Mn (manganese) were used as a base material and an activator,
respectively. Manganese is doped as a divalent cation.
<Preparation of Phosphors 1 and 2>
[0029] There were mixed 150 ml of an ethanol solution of zinc acetate (0.2 mol/l) and 25
ml of an ethanol solution of manganese acetate (0.012 mol/l), followed by being stirred
using a magnetic stirrer for 10 minutes while being kept at 10 °C in a temperature-controlled
cooling bath to obtain a mixed solution.
[0030] Subsequently, while being kept at 10 °C, 60 ml of a sodium sulfide aqueous solution
(0.45 mol/l) was prepared. While this solution was vigorously stirred in a temperature-controlled
bath at 10 °C, the mixed ethanol solution of zinc acetate and manganese acetate was
gradually added thereto using a rotary pump at a feed rate of 20 ml/minute. After
the addition, the reaction system was vigorously stirred for 10 minutes while the
temperature was controlled to 10 °C.
[0031] Further, using a sampling solution, phosphor particles formed were observed with
a transmission electron microscope (TEM), and then it was verified that the average
diameter of 100 particles was 5.0 nm. Still further, with regard to the particle diameter
size distribution, the standard deviation was obtained from the sizes of the 100 particles,
and then the monodispersion degree (%) was represented by the standard deviation/the
average particle diameter. The results confirm that the monodispersion degrees are
high, as shown in Table 1.
[0032] Subsequently, while stirring, 50 ml of acrylic acid as an organic coating material
was added using a rotary pump at a feed rate of 20 ml/minute, and after the addition,
the reaction system was vigorously stirred for 10 minutes.
[0033] This resulting solution was centrifuged using a centrifugal machine at 8,000 rpm
for 10 minutes to separate precipitates from nonreacted constituents. The precipitates
having been isolated were dried at 50 °C for 24 hours.
[0034] Phosphor 1 (ZnS:Mn) (corresponding to Example 1) obtained by pulverizing a dried
solid substance was evaluated as described later.
[0035] Herein, Phosphor 2 (corresponding to Example 2) was prepared in the same manner as
Phosphor 1 described above, except that no acrylic acid was added, and then evaluated
as described later.
[0036] Further, Phosphors 3 - 11 (corresponding to Examples 3 - 11, respectively), being
different in particle diameter and distribution, were prepared in the same manner
as for Phosphor 1, except that the concentration of zinc acetate, the concentration
of manganese acetate, and the concentration of sodium sulfide, as well as the temperature
and the feed rate were changed. In this case, Phosphor 6 with no acrylic acid added
was also prepared. Incidentally, via acrylic acid modification, a nanophosphor composite,
having a particle diameter 10% larger than that of the original nanophosphor, was
prepared.
<Synthesis of a phosphor for Comparison 4 (corresponding to Example 12)>
[0037] As a raw material for a base material, zinc acetate and manganese acetate are blended
at a mol ratio of 10 : 1 and mixed using a ball mill. Sulfur was added to this mixture,
followed by being placed in a sealed container for firing at 1,000 °C for 2 hours.
The resulting product was pulverized using a wet-type ball mill to obtain a finely
particulate phosphor, which was subjected to classification to give ZnS:Mn having
the particle diameter described in Table 1. Then the following evaluations were carried
out.
[Emission Luminance Distribution Evaluation]
[0038] The emission luminance per particle was determined with a near-field microscope (produced
by JASCO Corp.) using a light source of an excitation wavelength of 365 nm. Determination
was conducted for 100 particles to obtain the luminance dispersion ratio shown in
Table 1.
[Emission Luminance Evaluation]
[0039] As the emission luminance, luminance was determined using Konica Minolta LS-100 in
which 1 g of a phosphor was exposed to excitation light at 365 nm, being represented
by a relative value when that of Comparison 1 was designated as 100%.
[Half-value Width Evaluation of Light Emission]
[0040] Similarly to the above, the spectra of 100 particles were obtained using a spectrometer,
in which a synthesized particle of the present invention was exposed to a light source
at an excitation wavelength of 356 nm, and the emitted light therefrom was allowed
to pass through an optical fiber and detected by a photodiode. The spectrum width
at half the maximum emission intensity of the spectrum was evaluated. The average
value for the 100 particles is listed in Table 1.
[Durability Evaluation]
[0041] Continuous irradiation was carried out for 10 hours using a UV light source (254
nm). Luminance on 356 nm light excitation was measured, and represented as a relative
value (%) after UV irradiation based on a value measured before the irradiation.
Table 1
Example No. |
Average Particle Diameter (nm) |
Particle Diameter Dispersion degree (%) |
Luminance Dispersion Ratio (%) |
Emission Half-value Width (nm) |
Acrylic Acid (added/not added) |
Luminance |
Durability |
Remarks |
1 |
5.0 |
10 |
8.0 |
9.0 |
added |
140 |
100 |
Inv. |
2 |
5.0 |
10 |
8.0 |
9.0 |
not added |
133 |
97 |
Inv. |
3 |
3.0 |
10 |
6.0 |
10 |
added |
150 |
100 |
Inv. |
4 |
8.0 |
10 |
11 |
11.0 |
added |
130 |
100 |
Inv. |
5 |
4.0 |
15 |
9.0 |
10 |
added |
135 |
100 |
Inv. |
6 |
3.0 |
10 |
6.0 |
8.0 |
not added |
120 |
95 |
Inv. |
7 |
8.0 |
15 |
12 |
13 |
added |
130 |
97 |
Inv. |
8 |
2.0 |
5 |
5.5 |
6.5 |
added |
155 |
100 |
Inv. |
9 |
8.0 |
25 |
25 |
22 |
not added |
100 |
80 |
Comparison 1 |
10 |
3.0 |
25 |
20 |
25 |
not added |
95 |
75 |
Comparison 2 |
11 |
20 |
15 |
25 |
20 |
not added |
90 |
70 |
Comparison 3 |
12 |
10 |
35 |
40 |
35 |
not added |
60 |
30 |
Solid-phase Method Comparison 4 |
[0042] As shown in Table 1, it is understood that excellent emission luminance and durability
can be realized in such a manner that particle formation is controlled so that emission
luminance distribution may fall within a monodispersion degree which is the range
of the present invention, as described in the present invention. Although particle
diameter distribution greatly contributes to the above, inner defects are presumed
to be another factor.
[0043] Further, it is understood that in the nanophosphor composite of the present invention
whose surface is coated via addition of acrylic acid, emission intensity is greatly
enhanced as well as durability via the surface coating.